Face coverings are now mandatory in many places across the UK with the latest measures making it compulsory to don a mask in shops, hospitals, some leisure venues and public transport in England.
Similar rules are in place for Wales, Scotland and Northern Ireland.
But while the rules state that a covering just has to be something that masks your mouth and nose and wraps around the side of your face - experts have issued a warning over one particular type.
There have already been warnings that to be effective masks should be at least two - and possibly three layers thick.
Now the Centres for Disease Control and Prevention have urged people to avoid masks with valves, reports The Mirror.
It explained: “The purpose of masks is to keep respiratory droplets from reaching others to aid with source control.
“However, masks with one-way valves or vents allow air to be exhaled through a hole in the material, which can result in expelled respiratory droplets that can reach others.
“This type of mask does not prevent the person wearing the mask from transmitting COVID-19 to others.
“Therefore, CDC does not recommend using masks for source control if they have an exhalation valve or vent.”
